The present invention relates to platen variable arrangements used in a typewriter or the like having a platen that rotates about its axis. More particularly, it concerns apparatus for the selective control of rates of platen rotation either in coarse step increments or in refined continuous amounts less than the step increments.
Typewriters or the like having a rotatable platen typically include a platen variable mechanism that enables the operator to manually select and control the rate of platen rotation. The platen is rotatable by a coarse adjusting means in step increments or by a fine adjusting means in continuous amounts less than the step increments. The need for operation from the coarse adjusting means occurs during paper insertion, paper removal and rapid vertical paper advancement past a printing station in the typewriter. The coarse adjusting means typically comprises a pair of platen knobs supported coaxially with the platen. The step increment movement or coarse adjustment of the platen by the platen knobs is usually controlled by a sprocket or gear means having teeth copperable with a detent device of a linespacing mechanism. The step increments are detectably felt by the operator through the selected platen knob and are in step amounts corresponding to a particular line feed spacing. In addition to and in conjunction with the coarse adjusting means, the platen variable is provided with a mechanism to rotatably adjust the platen in refined amounts. The fine adjusting means is used to manually select the position of paper fed by the platen so as to bring previously typed work into alignment with the printing station. The latter mechanism operates to rotate the platen independently of the mechanism which provides the step increments of the coarse adjusting means.
Most prior art platen variables provide mechanisms to successfully accomplish platen adjustment in coarse and fine amounts. Further, the prior art employs platen knobs for manual rotation of the platen in coarse amounts. A platen knob is directly assembled at each extreme end of the platen in a straightforward manner for rotation therewith. The prior art discloses various mechanisms used to finely adjust the platen position.
A common approach to fine platen adjustment is to provide a mechanism that disengages the platen from the linespacing mechanism, thereby permitting free rotation of the platen. Such a fine adjusting mechanism includes a release button or the like which, when axially pushed or pulled, along the platen axis, decouples the linespace mechanism to thus allow free platen rotation via the platen knobs. This type of a fine adjustment mechanism is awkward to use as it necessitates axial movement of the release button while simultaneously rotating one or both of the platen knobs.
An improved design for a platen variable is disclosed in U.S. Pat. No. 3,461,997 granted on Aug. 19, 1969 to Donald R. Humphreys. This patent teaches a knob assembly comprising a coarse adjusting knob for incremental indexing of a platen in conjunction with a fine adjusting knob for a more refined platen adjustment. Each knob is independently manually operable. The manner in which a refined adjustment is accomplished is based on a known harmonic drive device disclosed in U.S. Pat. No. 2,906,143 granted Sept. 29, 1959 to C. Walton Musser. While the Humphreys patent is a successful improvement, the design as disclosed is undesirable for the reason that the manner of use for satisfactory operation is both confusing and unconventional. By way of example, two embodiments are disclosed in the Humphreys patent. The first embodiment includes the undesirable placement of the fine adjusting knob outboard from the coarse adjusting knob. This arrangement will tend to confuse the operator as to the intended function of each adjusting member. The second embodiment corrects this problem by reversing the locations of the adjusting members. However, the resulting motion of the platen in response to rotation of the adjusting knob members is highly unacceptable. Specifically this embodiment includes an arrangement of parts that provides for a rotation of the platen by the fine adjusting knob in a direction opposite to that which results when the same manipulation is applied to the coarse adjusting knob. There is then an inconsistency in the direction motion of the platen as effected by the coarse and fine adjusting knobs, respectively.